IVUS imaging procedures are widely used in interventional cardiology as a diagnostic tool for assessing a vessel, such as an artery, within the body of the patient to determine the need for treatment, to guide intervention, and/or to assess the effectiveness of administered treatment. An IVUS imaging system uses ultrasound echoes to form a cross-sectional image of the vessel of interest. Typically, IVUS imaging uses a transducer in a catheter to emit ultrasound signals (waves) and to receive the reflected ultrasound signals. The emitted ultrasound signals (often referred to as ultrasound pulses) pass easily through most tissues and blood, but they are partially reflected by discontinuities arising from tissue structures (such as the various layers of the vessel wall), red blood cells, and other features of interest. The IVUS imaging system, which is connected to the catheter by way of a patient interface module, processes the received ultrasound signals (often referred to as ultrasound echoes) to produce a cross-sectional image of the vessel proximate to where the catheter is located.
The two types of catheters in common use today are solid-state and rotational. A conventional solid-state catheter may use an array of transducers (typically 64) distributed around a circumference of a sheath, which is an outer layer of the catheter. The transducers are connected to an electronic multiplexer circuit. The multiplexer circuit selects transducers from the array for transmitting ultrasound signals and receiving reflected ultrasound signals. By stepping through a sequence of transmit-receive transducer pairs, the solid-state catheter can synthesize the effect of a mechanically scanned transducer element, but without moving parts. Since there is no rotating mechanical element, the transducer array can be placed in direct contact with blood and vessel tissue with minimal risk of vessel trauma, and the solid-state scanner can be wired directly to the IVUS imaging system with a simple electrical cable and a standard detachable electrical connector.
On the other hand, a conventional rotational catheter may include a flexible drive cable that continually rotates inside the sheath of the catheter inserted into the vessel of interest. The drive cable may have a transducer disposed at a distal end thereof. The transducer is typically oriented such that the ultrasound signals propagate generally perpendicular to an axis of the catheter. In the typical rotational catheter, the sheath may be filled with fluid (e.g., saline) to protect the vessel tissue from the rotating drive cable and transducer while permitting ultrasound signals to freely propagate from the transducer into the tissue and back. As the drive cable rotates (e.g., at 30 revolutions per second), the transducer is periodically excited with a high voltage pulse to emit a short burst of ultrasound. The ultrasound signals are emitted from the transducer, through the fluid-filled sheath and sheath wall, in a direction generally perpendicular to an axis of rotation of the drive cable (i.e., the axis of the IVUS catheter). The transducer then listens for returning ultrasound signals reflected from various tissue structures, and the IVUS imaging system assembles a two dimensional image of the vessel cross-section from a sequence of several hundred of these ultrasound pulse/echo acquisition sequences occurring during a single revolution of the drive cable and the transducer.
However, the images obtained by the conventional rotational catheters exhibit distortion caused due to non-uniform rotational distortion (NURD). The distorted images fail to provide the required insight into the vessel condition. NURD may occur due to, for example, friction between the drive cable and the sheath that encloses the drive cable; friction between the sheath and the vessels through which the catheter travels through during use; non-symmetrical drive cable/transducer assembly that causes the drive cable to resist bending more at some angles than at other angles (when rotated, these asymmetries cause the drive cable to store more energy in some angular orientations and then to release that energy as the drive cable is rotated past that angle); the sheath and drive cable containing various bends and twists along its path to the vessel of interest, resulting in the transducer rotating at a non-uniform angular velocity even though the drive cable is rotated at a constant speed. As such, the conventional rotational catheters fail to adequately minimize non-uniform rotational distortion, while also providing sufficient strength and flexibility.
Intravascular devices are also used diagnostically to measure internal body dimensions, estimate lesion lengths, and to ensure accurate placement of the intravascular device within the body of the patient. In several of these procedures, it is advantageous to be able to visualize the progress of the transducer, which enables the imaging, towards the target location within the body of the patient. Introducing intravascular devices into the body often requires fluoroscopic visualization to aid the treating healthcare provider in guiding the intravascular device to the target site. Intravascular devices are commonly formed of a non-radiopaque polymeric material. Therefore, radiopaque markers are affixed at a distal end of the intravascular device to enable the intravascular device to be visualized during x-ray and fluoroscopic procedures. For example, in intravascular procedures, health care providers may guide the intravascular device to a target location by using fluoroscopy to track the position of radiopaque markers on distal end of the intravascular device.
Conventionally, these radiopaque markers are circumferential metallic bands affixed to the exterior surface of the intravascular device. Although these marker bands allow the distal end of the intravascular devices to be visualized by fluoroscopy, they present certain problems. First, affixing of the radiopaque markers to the exterior surface of the sheath of the catheter fails to provide guidance as to the accurate position of the included transducer that enables the imaging within the body of the patient. The operator is undesirably left to guess or estimate the position of the transducer during the imaging procedure. Further, metallic marker bands require affixing (e.g., by crimping, swaging, or adhesive) to the underlying intravascular device to avoid slippage as the intravascular device is moved through the body. The bands may protrude from the tubular surface of the intravascular device and increase the intravascular device profile, which creates frictional resistance to the translational movement of the intravascular device through body passages, and potentially damages tissues contacting the moving intravascular device. In some instances, where a marker band has been swaged onto the exterior surface of the intravascular device, and the inner diameter of a marker band is greater than the outer diameter of the intravascular device, buckling may occur, causing the marker band to crack and the exterior surface to tear. Finally, the placement of band markers on the outer surface presents problems with inadvertent disassociation of the markers from the intravascular device, with attendant loss of positional and measurement accuracy. In addition, such marker bands are constructed from expensive and heavy radiopaque metals such as gold, platinum, tantalum, and alloys of these dense materials. The use of these heavy materials typically results in inflexible and rigid marker bands that can impair the trackability of the catheter by increasing the stiffness of the intravascular device, thereby compromising the flexibility and maneuverability of the intravascular device. As such, the conventional intravascular device with radiopaque markers affixed to the exterior surface fail to enable accurate measurement of internal body dimensions, accurate estimation of lesion lengths, and accurate placement of the transducer within the body of the patient.
Accordingly, there remains a need for improved ultrasound intravascular devices for use in IVUS imaging and associated devices, systems, and methods. The devices, systems, and methods proposed in the present disclosure overcome one or more of the deficiencies of conventional intravascular devices.